The failure of mechanical articles, or components, by fatigue cracking can occur if the article, or components, are exposed to cyclic loading or vibrations. The failure of a fan blade of a turbofan gas turbine engine by fatigue cracking is unacceptable and cracks must be detected before they reach a size at which they could cause the fan blade failure. The fan blades of some turbofan gas turbine engines are hollow, and the cracks are most likely to form through the walls defining the hollow chamber in the fan blades. These hollow fan blades are evacuated during the manufacturing process.
A known method of detecting cracks in hollow fan blades is to place a piezoelectric transducer on the surface of the hollow fan blade such that it is acoustically coupled to the hollow fan blade. The piezoelectric transducer is electrically excited to generate ultrasound at a particular frequency, for example 150 kHz, in the hollow fan blade. The piezoelectric transducer is then used to detect the ultrasound in the hollow fan blade and an analyser is used to monitor the decay rate of the amplitude of the ultrasound at the particular frequency. It has been found that the decay rate of the amplitude of the ultrasound at the particular frequency is proportional to the pressure in the hollow fan blade. Thus if there is a crack in the hollow fan blade the pressure is greater in the hollow fan blade than for an uncracked fan blade and therefore the decay rate for the cracked hollow fan blade has a higher decay rate than for an uncracked hollow fan blade.
The inspection of a full set of hollow fan blades on a turbofan gas turbine engine is extremely time consuming. The inspection of the hollow fan blades requires an inspector to couple the piezoelectric transducer to, and to test, each hollow fan blade in turn.
The prior art method and apparatus for detecting these pressure within the hollow fan blade is not automatic, is time consuming and requires contact between the detection apparatus and the hollow fan blade.